A simplified block diagram of an EUV lithography system is shown in FIG. 1 (PRIOR ART). The EUV source 102 at 13.5 nm is normally a hot plasma the emission of which is collected by the collector 104 and delivered to an illuminator 106 through an intermediate focus (IF). The latter illuminates a mask or reticle 108 with the pattern to be transferred to the wafer 110. The image of the mask or reticle is projected onto the wafer 110 by the projection optics box 112. The main purpose of the collector 104 is to deliver the maximum amount of in-band power from the source 102 to the illuminator 106, by matching the constraint due to the source 102 and the illuminator 106 itself. The ratio between the total EUV power available at the IF and the total power EUV power emitted by the source 102 in 2π sr is called collection efficiency. The primary goal of the optical design of the collector 104 is to maximize the collection efficiency while matching the boundary constraints due to the source 102 and the illuminator 106.
For preferred applications, the EUV radiation has a wavelength of the order 13.5 nm, and for this, two types of plasma sources are currently considered as possible solutions to generate the radiation: Discharge Produced Plasma (DPP) sources and Laser Produced Plasma (LPP) sources. In the former case, the plasma is generated by means of an electric discharge through the fuel (e.g. Sn, Xe, etc.), whereas in the latter case, the plasma is produced by a laser beam impacting on a fuel target (e.g. Li, Sn, Xe, etc.).
For an LPP source, the conventional configuration of choice is shown in FIG. 2 (PRIOR ART); see US 2005/0199829 Al. A single elliptical (collector) mirror 104 is placed behind the source 102. In the standard configuration, the laser beam 204 reaches the fuel target at the source (focus) 102 from the side of the mirror 104 opposite to the IF, through a hole 206 in the center of the mirror 104. The incidence angles on the mirror 104 are normally smaller than about 30°, and a multilayer coating (normally consisting of a Mo and Si stack) is required to assure enough reflectivity at 13.5 nm. Mirrors with small incidence angles will be referred to herein as Normal Incidence Mirrors (NIM). The configuration of FIG. 2 is the most efficient. However, this configuration has several drawbacks—                1) The elliptical NIM of FIG. 2 requires a complex and expensive Mo/Si multilayer coating.        2) The surface area of the NIM is relatively small and consequently the thermal load density is very high. This makes the thermal control of the NIM challenging.        3) Since plasma sources emit most of their power between about 10 nm and 120 nm, and since the multilayer coating acts as a very narrow pass-band filter reflecting only a very small bandwidth around 13.5 nm, the total thermal load is higher on the NIM. This further complicates the thermal control of a NIM.        4) The configuration of FIG. 2 prevents the use of Debris Mitigation Tools (DMT) commonly used with DPP sources. This type of DMT consists of many thin lamellas radially placed around the source and filled with a gas (e.g. Ar, N2, etc.) to slow down fast particles from the source. However, this type of DMT would completely block the radiation in the configuration of FIG. 2.        5) To overcome the erosion of the reflective layer by high-energy ions and particles from the source and to extend the lifetime of the mirror, the thickness of the reflective layer needs to be increased. However, in a MM this implies increasing the numbers of layers in the multilayer coating, also increasing the complexity and costs of the manufacturing process.        6) The ion and particle flow (per unit area) is relatively high in a NIM, due to its small area. Consequently, the erosion and/or deposition is relatively high in a NIM.        
The present invention seeks to address the aforementioned and other issues.